Las Vegas: A stony-faced Floyd Mayweather issued a warning to Conor McGregor on Wednesday as the two fighters faced off just three days ahead of their money-spinning boxing duel.

In stark contrast to their lurid, trash-talking global press tour last month, a subdued McGregor and a stern-looking Mayweather spoke respectfully as they prepared for Saturday's 12-round boxing contest at the T-Mobile Arena.

Floyd Mayweather vs Conor McGregor face-off for the final time before the fight on 26 August. Image Courtesy: Twitter @ShowtimeBoxing

Mayweather, who bombarded McGregor with expletives and homophobic slurs during last month's frenzied publicity blitz, did not swear once during Wednesday's news conference.

McGregor, the massive underdog for this weekend's bout, restricted himself to verbally abusing a heckler in the audience who predicted he would be knocked out.

McGregor faces his first ever professional boxing contest against Mayweather, the undefeated former welterweight king who has come out of a two-year retirement to take on the Irish mixed martial arts star.

However McGregor, the 29-year-old two-time UFC world champion from Dublin, insisted he was ready to stun the world of combat sports by upsetting Mayweather.

"Been to a lot of these crazy press conferences," said McGregor. "This is a lot more subdued. More business-like, the way I like it."

"We are prepared for 12 three-minute rounds of non-stop pace. We are prepared, I will put pressure on him and break this old man. Trust me on that.

"I don't see him lasting two rounds. I feel I have the decision to end it inside one."

'I'll die a fighter'

Mayweather, 40, looked relaxed throughout, even taking time to quietly admonish members of his entourage who had shouted at McGregor.

And rather than the abuse of last month, Mayweather praised McGregor as a "helluva fighter, a stand-up guy and a tough competitor."

"It's not going to be an easy fight," Mayweather said. "It's going to be blood sweat and tears. "He's the best at what he do, I'm the best at what I do. It comes down to the two competitors going out there and displaying our skills."

But Mayweather's words came with a warning for McGregor.

"After 21 years I've been hit with everything and I'm still right here," Mayweather said.

"And if you give it, you must be able to take it. There have been plenty of guys who talked a lot of trash, but when it's all said and done, I came out victorious.

"I've said it's not going the distance and you can mark my words.

"Anything and everything in boxing that can be done, I've done it. I was born a fighter, I will die a fighter."

McGregor is hoping his punching power can catch Mayweather out on Saturday. But the American veteran warned he had faced plenty of hard hitters before.

"Manny Pacquiao got bombs, Canelo (Alvarez) got bombs, Shane Mosley got bombs. But remember this — I got a great chin. And the same way you give it, you've got to be able to take it."

McGregor meanwhile laughed off suggestions from his opponent earlier this week that he may struggle to make the 154-pound limit at Friday's weigh-in.

"I've never missed weight in my life and it won't be happening this time," McGregor said. "I've had my nutritionist practically living with me throughout camp."

The showdown between Mayweather and McGregor was confirmed in June after a protracted guessing game about whether the two fighters from different sports would ever meet.

Saturday's bout could become the most lucrative fight in history according to Showtime Sports television executive Stephen Espinoza.

Mayweather could add another $200 million to his career earnings depending on pay-per-view sales while McGregor, who only four years ago was a struggling former plumber's apprentice living on welfare, could collect $100 million.

This will be the most widely distributed pay per view in history," Espinoza said.

"That's not hyperbole, that's fact. We are well on the way to a record-setting event."